The present invention relates to a modem that encodes a voice signal, converts the encoded voice signal to a passband signal, transmits the passband signal onto an analog transmission line, receives the passband signal from the analog transmission line, converts the passband signal to a code, and then decodes it into a voice signal. Such techniques are essential for voice multiplexing modems designed to transmit voice signals together with data signals over a single analog transmission line.
In transmitting only voice over an analog transmission line, it is only necessary to send the voice signal in its original analog form over the transmission line. However, in the case of a voice/data multiplexing modem, at the transmitting end, the analog voice signal is first converted to a multi-bit code and then supplied to a transmitter together with a data signal. In the transmitter of a conventional modem, the input digital signal is modulated and converted to a passband signal for transmission over the analog line. At the receiving end, a receiver receives the passband signal from the analog line and then demodulates it into a digital data, as in the case of a receiver in a conventional modem. An encoded voice contained in the digital signal which is output from the receiver is decoded by reversing the process performed at the transmitting end, and the voice signal is thus reproduced.
Conventional modems momentarily can be thrown into a diverging condition (an out of synchronization condition) at the time of line switching or at the time of switching to a connection for loop testing. At this time, since the received signal does not remain stationary in the complex plane, random data is output from the modem receiver until after synchronization is achieved again. In this case, a voice signal is reproduced as a jarring noise having a large sound volume. Such a sound level is very unpleasant when heard through a telephone receiver.
It is accordingly an object of the present invention to provide a voice/data multiplexing modem that does not output a jarring noise when thrown into a diverging condition.
These and other objects are met in accordance with the present invention which is directed to modem including a receiver section that receives an analog signal from an analog transmission line and outputs code. The analog signal is a signal converted from an encoded voice signal that is created by mapping a voice signal to a set of codes that excludes a particular code. A decoder section that restores the voice signal from the code output from the receiver section. A detector section that detects the particular code from the code output from the receiver section. A masking section that masks an output of the decoder section when the detector section has detected the particular code.